Photoconductive film



Sept. 1, 1970 I AKIR sAsANQ ETAL 3,526,543

PHOTOCONDUCTIVE FILM Filed Jan. 51, 1967 INVENTORS fine/2n dives/v01/7811? Om'mun ATTORNEY United States Patent O 3,526,543 PHOTOCONDUCTIVEFILM Akira Sasano, Kokubunji-shi, and Mitsuru Oikawa, Tokyo, Japan,assignors to Hitachi, Ltd., Tokyo, Japan, a Japanese corporation FiledJan. 31, 1967, Ser. No. 612,866 Claims priority, application Japan, Feb.28, 1966, 41/ 11,678 Int. Cl. B44d 1/18 US. Cl. 117-217 7 ClaimsABSTRACT OF THE DISCLOSURE A photoconductive film to be used as a targetof a vidicon and the like, said film being formed by laminating a filmlayer of metal and a photoconductive layer upon a conductive transparentlayer and characterized in that the photoelectric current is increasedby reducing the dark current.

This invention relates to an improvement in a photoconductive film to beused as a target of a vidicon and the like, and it is an object of theinvention to provide a sensitive photoconductive film wherein thephotoelectric current is increased by reducing the dark current.

Other objects, features and advantages of the invention will becomeapparent from the following detailed description of the invention whentaken in conjunction with the accompanying drawings, in which:

The drawing is a sectional diagram of photoconductive film embodying thepresent invention.

A conventional photoconductive film used as a target of a visual vidiconhas been formed by laminating a conductive transparent layer and aphotoconductive layer on a glass substrate which constitutes a vidicontube. In this apparatus, the variation in the electrical conductivity ofthe photoconductive layer caused by the incident light is converted tothe variation in the current of the radiating electron beam, and saidvariation is detected as an output current by the conductive transparentlayer which works as a signal electrode. Among the photoconductive filmsof this kind, a film which employs a NESA film (SnO trademark) as theconductive transparent layer 2 and Sb S film as the photoconductivelayer has a transparent signal electrode and enjoys a high electricalconductivity. Therefore, this photoconductive film works rather well.However, since the dark current is large in this device, it isimpossible to increase the photoelectric current.

In order to improve said defect, an attempt has been made to employ asemitransparent evaporated metal layer as the signal electrode, butsince the transverse electrical conductivity is required for the signalelectrode, the layer must be thick. This, in turn, lowers thetransparency of the layer and thus it has been difficult to put saidlayer to practical use.

The present invention is intended to obviate the deficiencies describedabove.

The present invention consists of a photoconductive film formed bylaminating a film layer of metal and a photoconductive layer on aconductive transparent layer.

The drawing shows an example of the photoconductive film according tothe invention, wherein reference numeral 1 designates a glass substrate,2 a conductive transparent layer, 3 a film layer and 4 a photoconductivelayer. Good results were obtained with this device when the conductivetransparent layer 2, the film layer 3, the photoconductive layer 4 weremade to be 300-600 A., -100 A. and 3-5 in thickness, respectively.

The existence of said film layer 3 changes the formof the energy bandsof the photoconductive body by the contact thereof with thephotoconductive layer and ac- 3,526,543 Patented Sept. 1, 1970 "icecordingly, by suitably selecting the metal to be used for the filmlayer, the contactness with the photoconductive layer can be freelychanged. Thus, the electrical resistance of the photoconductive layercan be increased and thereby the characteristics of the photoconductivelayer can be improved.

In a photoconductive film formed in this way, both the film layer 3 andthe conductive transparent layer 2 work as a signal electrode and, inaddition, the conductive transparent layer 2 has a high electricalconductivity. Therefore, the film layer 3 may not have a high electricalconductivity and so a thin evaporated film or the like can be used assaid film layer. Thus, the reduction of the transparency against theincident light can be minimized.

The metals most used for the film layer are Ag, Al and Cu and they areparticularly suitable when the photoconductive layer is formed of As SeAS28623, As SeS AS2S3, or Se.

Now, the embodiments of the invention will be described.

EMBODIMENT 1 As the film layer, Ag is evaporated on a NESA film adheredto a glass substrate. Upon said film layer is evaporated As Se to form aphotoconductive layer and thus a photoconductive target is provided.When this target is used with a target voltage of 60 v., the darkcurrent is 0.005 ,ua. and the photoelectric current is 0.2 u'a. with anillumination of 2.5 luX. On the other hand, in a conventional targetconsisting of a NESA film and an evaporated As Se film, the dark currentis 0.03 a and the photocurrent is 0.1 [1.21. with an illumination of 10lux when said conventional target is used with a target voltage of 5 v.Thus, the reduction of a dark current and the increase of a photocurrentare achieved.

EMBODIMENT 2 As the film layer, Al is evaporated on a NESA film adheredto a glass substrate. On said film layer is evaporated As Se to form aphotoconductive layer and thus a photoconductive target is provided.When this target is used with a target voltage of 20 v., the darkcurrent is 0.004 a. and the photoelectric current is 0.3 ,ua. with anillumination of 2.5 lux. Also in this case, comparison with the knowntarget as described in Embodiment 1 shows that a good result is obtainedwith this novel target.

EMBODIMENT 3 As the film layer, Al is evaporated on a NESA film adheredto a glass substrate. On said film layer is evaporated As Se S to form aphotoconductive layer and thus a photoconductive target is provided.When this target is used with a target voltage of 50 v., the darkcurrent is 0.002. ,ua. and the photocurrent is 0.1 p.21. with anillumination of 2.5 lux. On the other hand, when a conventional targetconsisting of a NESA film and an evaporated As Se S film is used with atarget voltage of 10 v., the dark current is 0.012 a. and thephotoelectric current is 0.08 a. with an illumination of 10 lux. Thus,evidently the reduction of a dark current and the increase of aphotocurrent are achieved.

EMBODIMENT 4 As the film layer, Al is evaporated on a NESA film adheredto a glass substrate. On said film layer is evaporated As Se S to form aphotoconductive layer and thus a photoconductive target is provided.When this target is used with a target voltage of 50 v., the darkcurrent is 0.002 ,aa. and the photoelectric current is 0.05 a. with anillumination of 2.5 lux. On the other hand, when a conventional targetconsisting of a NESA film and an evaporated As Se S film is used with atarget voltage of 30 v., the dark current is 0.01 ,ua. and thephotocurrent is 0.05 a. with an illumination of lux. Thus, the reductionof a dark current and the increase of a photocurrent are achieved.

It is to be noted that the increase in dark current due to the passageof time, as seen in a conventional target consisting of a NESA film andan evaporated film of As Se As Se S, As SeS or the like, is not found inthe photoconductive target according to Embodiments l to 4. This fact isalso preferable for increasing the sensitivity of a photoconductivelayer. Further, though the embodiments using Al or Ag for the film layerare described hereinabove, other kinds of metal like Cu can also be usedto obtain the same effect.

As is fully described hereinabove, the photoconductive film according tothe invention can reduce the dark current present at a conventionalphotoconductive target consisting of a conductive transparent layer andphotoconductive layer. Accordingly, it has become possible to increase atarget voltage and hence to increase the photoelectric current. Also,the increase with time of the dark current can be prevented.

Further, since both the conductive transparent layer and the film layerWork as a signal electrode in the photoconductive film according to theinvention, the contactmess with the photoconductive layer can beadjusted by suitably selecting the film layer. The invention can equallybe applied to the case where As S Se or the like is used for thephotoconductive layer. Thus, the present invention makes it possible tofabricate a photoconductive film having good characteristicsunattainable by a conventional film consisting of a conductivetransparent layer and a. photoconductive layer, and hence it has becomepossible to provide a highly eflicient vidicon. Therefore, the presentinvention has a wide range of industrial applications.

What is claimed is:

1. A photoconductive film consisting essentially of a substrate, aconductive transparent layer of a material which is more transparentthan a correspondingly conductive semitransparent metal layer formed onsaid substrate, a metal film selected from the group consisting of Al,Ag, and Cu films on said conductive transparent layer, and aphotoconductive layer consisting of a material selected from the groupconsisting of As Se As Se S, As SeS and As S said metal film providing alayer of high electric resistance as a result of the contact thereofwith said photoconductive layer.

2. A photoconductive film as defined in claim 1, wherein said metal filmcomprises Al and said photoconductive layer is As Se 3. Aphotoconductive film as defined in claim 1, wherein said metal filmcomprises Ag.

4. A photoconductive film as defined in claim 1, wherein said metal filmcomprises Cu.

5. A photoconductive film as defined in claim 1, wherein the thicknessof (a) the conductive transparent layer is about 300 600 angstroms, (b)the metal film is about 10100 angstroms and (c) the photoconductivelayer is about 3-5;.

6. A photoconductive film consisting essentially of a substrate, aconductive transparent layer comprising SnO formed on said substrate, ametal film selected from the group consisting of aluminum, silver andcopper formed on said conductive transparent layer, and aphotoconductive layer consisting of a material selected from the groupconsisting of As Se As Se S', As SeS and AS253, said metal filmproviding a layer of high electric resistance as a result of the contactthereof with said photoconductive layer.

7. A photoconductive film as defined in claim 6, Wherein the thicknessof (a) the conductive transparent layer is about 300-600 angstroms, (b)the metal film is about 10-100 angstroms and (c) the photoconductivelayer is about 3-5 References Cited UNITED STATES PATENTS 2,822,300 2/1958 Mayer et al 117-4201 2,833,675 5/1958 Weimer 117-201 2,844,5437/1958 Fotland 117201 3,350,595 10/1967 Kramer 117217 ALFRED L. LEAVITT,Primary Examiner C. K. WEIFFENBACH, Assistant Examiner US. Cl. X.R.117227; 313-94

